Eastern margin of the Sea of Japan

The Sea of Japan represents a back-arc basin that formed via geological rifting of continental crust from the late Oligocene to middle Miocene (28–13 million years ago).

[4] Following the end of seafloor spreading, its eastern margin experienced weak compression between 10 and 3.5 million years ago.

Crustal shortening has been ongoing in the eastern margin and back-arc region of the Northeastern Japan Arc since 3.5 million years ago.

This deformation is attributed to east–west compressive forces, forming fold and thrust belts along the eastern margin.

[6] The basis for defining this tectonic boundary is the occurrence of large magnitude 7 or greater earthquakes along a linear zone from offshore Niigata Prefecture to off the west coast of Hokkaido.

[11] The associated geological faults related to the opening of the sea and compressive forces along the eastern margin influence seismic activity.

[54] Due to their closer proximity to land, the accompanying tsunamis require a shorter time arriving compared to earthquakes along Japan's Pacific coast.

Another seismic gap of 50 km (31 mi) length is thought to exist off the coast of Hokkaido between the 1940 and 1993 rupture zones.